The present invention relates to a color projection display apparatus, and more particularly to a color temperature corrector in use with a projection display apparatus, which is well suitable for a multi-screen used in studios and theaters.
The projection display apparatus is used for projecting a video image, such as a television image, VTR reproduction image, and an image by a laser-vision disk (LVD), on a viewing screen in an enlarged fashion.
Two types of the projection display apparatuses are known such as a 3-tube color projection display apparatus and a liquid-crystal color projection display apparatus. The former uses three projection tubes (high brightness CRTs) for projecting three different mono-colors, red (R), green (G), and blue (B). The latter projects light emitted from an incandescent light source through liquid-crystal shutters on a viewing screen, to form an image on the screen. In the projection structure, the projection display apparatus consists of two types of display apparatus, a transmitting screen type and a reflecting screen type.
The projection display apparatus is used in studios and theaters, and at home as well. When the projection display apparatus is used in a studio or a theater, correction of a color temperature of the projection display apparatus becomes problematic. The light in a studio, for example, usually consists of artificial lights emitted from many types of illumination lamps, not natural light. Accordingly, a color temperature of the screen of the projection display apparatus is not always proper value. Therefore, the color temperature correction is required. Particularly where an image on the screen of the projection display apparatus, together with a human figure, for example, are picked up with a TV camera, one encounters a problem that when the image picked up is reproduced on a TV screen, the coloring of the reproduced image is indefinably unnatural. This results from the fact that the color temperature characteristic of the TV camera has been set in connection with the human figure alone. Therefore, to obtain the reproduced image of natural coloring, the color temperature of the projection display apparatus must be corrected with relation to the color temperature characteristic of the TV camera.
In a studio, a multi-screen is frequently used for the display screen of the projection display apparatus. The multi-screen is a large screen consisting of a number of projection display apparatus PJ, which are arrayed in a matrix fashion in a plane, as shown in FIG. 1.
In such a multi-screen, the color temperature correction must be applied to all of the projection display apparatus PJ similar to the above-described single screen projection display apparatus.
To correct the color temperature, a color temperature corrector is used. Three major systems of correcting the color temperature that have been known, a color corrector system, a colored film system, and an anode voltage control system, will be described.
1) Color Corrector System
This system corrects color tone and color distortion of a video signal derived from a video source (VS), such as an LVD and a VTR, by means of a color corrector. As shown in FIG. 2, a color corrector CC demodulates a video signal from a video source into R, G and B signals, corrects the signal levels of those signals, modulates them again into a corrected video signal, and sends it to a projection display apparatus PJ. On the basis of the corrected video signal, the apparatus PJ projects an image of a proper color temperature on a screen.
2) Colored Film System
This system is generally used in a projection display apparatus of the transmission screen type. To make the color temperature correction, as shown in FIG. 3, a colored film F, which has an absorption characteristic for color temperature correction, is attached to the front face of a screen S of a projection display apparatus PJ.
3) Anode Voltage Control System
This system is generally used in a projection display apparatus of the three-tube type. For the color temperature correction, the anode voltages of the projection CRTs are controlled in accordance with the parameters for correcting the color temperature, so that the amounts of lights emitted from the CRTs are controlled.
However, the color temperature correcting systems as mentioned above have the following disadvantages.
In the case of the color corrector system, adjustment of the color corrector is needed in addition to the adjustment of the projection display apparatus PJ. Further, when it is used in the multi-screen type shown in FIG. 1, the number of the correctors CC as required is equal to that of the projection display apparatus PJ. This results in increase of the cost to manufacture, and increase of the number of adjusting points.
In the case of the colored film system, a colored film of a large area is required because the screen per se of the projection display apparatus has a large area. Practically, it is very difficult to apply a colored film F over the screen S smoothly and uniformly or without wrinkles. The number of color temperature that one colored film can correct is only one. Therefore, to correct different color temperatures, a required number of colored films is equal to that of the color temperatures to be corrected. In case where the colored film system is applied to the multi-screen shown in FIG. 1, and the colored films are put on the screens of the projection display apparatus PJ, respectively, a variation in the optical characteristics of those films must be removed. Alternatively, those screens of the apparatus PJ may be covered with a single colored film. In this case, the film must be extremely large.
In the case of the anode voltage control system, the anode voltages must be varied in accordance with ambient conditions including ambient lights in a place where the projection display apparatus is installed. Further, it is difficult to standardize the color temperature correcting parameters. Additionally, it is difficult to stabilize high voltages of the anodes, e.g., about 30 kV. Further, when the projection display apparatus of the 3-tube type in which the power source for the electron guns of the tubes are connected in series, it is impossible to adjust the anode voltages of the three tubes individually. To individually adjust the anode voltages, the power sources must be arranged in parallel. This results a complicated circuit. In the case of the anode voltage control system, the color temperature may be indeed corrected by controlling the anode voltage, but a linearity of the color temperature correction becomes poor.